Radar systems typically include a number of radiating elements often in an array. The recent trend is to increase the number of radiating elements in an attempt to attain better performance. There is a relationship between the number of radiating elements in a phased array and system performance with regard to gain, beam-steering, ECCM (electronic counter-counter measures, for example, anti-jamming), null-steering, and advanced beam forming capability. The result is often a larger size array which increases the complexity of signal routing, heat management, transportation of the array to its intended location, and the like. When the size of the array is reduced to address these concerns, the radiating elements are placed closer together. The result is an interaction between adjacent radiating elements. Coupling (e.g., cross-talk) across adjacent radiating elements causes significant performance degradation including radiation pattern distortion and scan blindness. Indeed, the interaction between the resonating elements increases on the order of the inverse square of the separation distance.
The article “Metamaterial Insulator Enabled Superdirective Array,” by Buell et al., (IEEE Transactions on Antennas and Propagation, Vol. 55, No. 4, April 2007), incorporated herein by this reference, describes a metamaterial isolator including a unit cell made of a dielectric with the face having a planar metallized (e.g., copper) spiral. A number of these unit cells are stacked together serving as an isolating wall between adjacent radiating elements in an effort to block electromagnetic energy from being transmitted from one radiating element to the other. The result was a fairly narrow band gap isolating region (for both transmission and reflection) between adjacent radiating elements. Furthermore, each individual unit cell had to be aligned to an adjacent unit cell which created a need for accurate alignment and the potential for modified behavior arising from the air gaps between the unit cells. Addressing the latter problem requires the use of a polymeric filler material that exhibits the same electromagnetic properties as the substrate. The proposed technique also requires surface machining of the substrate containing the radiating elements and corresponding feed networks. The added steps associated with integrating individual unit cells adds to the cost and complexity of a system-level solution. Finally, the metallization constituting a resonator loop was constrained to a single vertical plane.
Chiu et al. in “Reduction of Mutual Coupling Between Closely-Packed Antenna Elements,” IEEE Transactions on Antennas and Propagations, Vol. 55, No. 6 (June 2007) proposes a new ground plane structure in an attempt to reduce mutual coupling between closely-packed antenna elements. One disadvantage of such a technique is a narrow band and a solution useful for only very narrow element spacing. Rajo-Iglesias et al. in “Design of a Planer EBG Structure to Reduce Mutual Coupling in Multilayer Patch Antennas,” 2007 Loughborough Antennas and Propagation Conference, (Apr. 2-3, 2007), proposed a relatively large embedded single-layer electromagnetic band gap structure which also exhibited a narrow band width. Fu et al. in “Elimination of Scan Blindness in Phase Array of Microscript Patches Using Electromagnetic Band Gap Materials,” IEEE Antennas and Wireless Propagation Letters, Vol. 3, (2004) proposed an electromagnetic bandgap (EBG) structure which required very large isolators and a specialized dielectric material. Donzelli et al. in “Elimination of Scan Blindness in Phased Array Antennas Using a Grounded-Dielectric EBG Material,” IEEE Transactions on Antennas and Propagation, Vol. 6, (2007) proposes a grounded-dielectric EBG substrate which exhibited a narrow bandwidth and a complicated and expensive substrate design. Chen et al. in “Scan Impedance of RSW Microstrip Antennas in a Finite Array,” IEEE Transactions on Antennas and Propagation, Vol. 53, No. 3 (March 2005) disclosed shorted annular rings incorporated into an antenna patch used to reduce surface waves and scan variation but were limited to 20° scanning and required large element spacing, and fairly large elements.